Composite structure for boiling liquids and its formation

ABSTRACT

A composite metal structure for boiling liquids comprising a smooth surface metal substrate and a cover sheet bonded to the substrate with sub-surface cavities and spaced restricted openings extending through the cover sheet being joined to the cavities.

United States Patent [151 3,684,007 Ragi [4 Aug. 15, 1972 1541 COMPOSITESTRUCTURE FOR [56] References Cited gggifig gg AND ITS UNITED STATESPATENTS 1,228,816 6/1917 Peterson et a1. ..126/387X [m Invenmr' Gmge sAmherst 3,384,154 5/1968 Milton ..165/133 x Assignee: Union CarbideCorporation, New

York, NY.

Filed: Dec. 29, 1970 App1.No.: 102,387

US. Cl ..165/133, 29/1573 R, 62/527,

165/180 1111. c1. .1281 13/00 Field of Search ..165/l33, 105, 180, 181;

Primary Examiner-Albert W. Davis, Jr.

Attorney-Paul A. Rose, Harrie M. Humphreys, John C. LeFever and LawrenceG. Kastriner [5 7] ABSTRACT A composite metal structure for boilingliquids comprising a smooth surface metal substrate and a cover sheetbonded to the substrate with sub-surface cavities and spaced restrictedopenings extending through the cover sheet being joined to the cavities.

16 clams, 10 Drawing Figures PATENTEDAums I372 I 3. 684' 007 sum 1 0r 6I F/GZZ.

INVENTOR ELIAS G. RAGI ATTORNEY PATENTEDMJB 1 m2 3.684; 007 sum 2 OF 6INVENTOR ELIAS (G RAGI ATTORNEY BACKGROUND OF THE INVENTION Thisinvention relates to composite metal structures for improving heattransfer from heated surfaces to boiling liquids and in particularstructures which enhance nucleate boiling. The invention also relates toa method for forming such a composite structure.

' It is known that structures for improved nucleate boiling may beprepared by bonding metal powders onto metal substrates to forminterconnected pores with restricted openings to the outer surfacehaving equivalent pore radius less than about 6 mils, as for exampledescribed in US. Pat. No. 3,384,154 to R. M. Milton. It is also knownthat improved nucleate boiling structures may be prepared by cuttingclosely spaced grooves in a metal wall with the outer ridges partlydeformed into the grooves so that sub-surface cavities result withrestricted openings through the outer ridges, as described in US. Pat.No. 3,454,081 to L. C. Kun and A. M. Czikk. Disadvantages of such priorart nucleate boiling structures are either the relatively high cost ofmachining equipment or the inability to form relatively large restrictedopenings (preferred for boiling liquids characterized by high Kelvinparameters) from metal powder.

An object of this invention is to provide a highly efiicient butinexpensive nucleate boiling heat transfer structure.

Another object is to provide an inexpensive method for preparing ahighly efficient nucleate boiling heat transfer structure. Other objectsand advantages of this invention will be apparent from the ensuingdisclosure and appended claims.

SUMMARY This invention relates to an improved composite metal structurefor nucleate boiling heat transfer to liquid from a heated surface andto a method for preparing such a composite metal structure.

According to this invention a composite metal structure comprises animpervious smooth surface metal substrate and an impervious metal coversheet bonded to the metal substrate and having at least 25 cavities perinch of substrate surface with each cavity having an effective diameterof at least 0.003 inches, and a multiplicity of spaced restrictedopenings extending through the cover sheet in fluid communication withthe cavities and having effective diameter such that the effectivediameter ratio of restricted openings to cavities is less than 0.8. Asused herein, the effective diameter of the cavity is the diameter of thelargest sphere which can be fitted within the confines of the cavity.The effective diameter of the restricted opening represents the largestdiameter vapor bubble which may emerge from the cavity to the outersurface of the cover sheet. For example, the effective diameter may bethe minor dimension of an ovoid or elliptically shaped restrictedopening.

Another aspect of this invention relates to a method for forming onecomposite metal structure of this invention. According to this method, adie having spaced pyramid shaped projections is pressed against a thinmetal foil backed by a resilient surface to initially form discretepyramid shaped sections in the foil. The

pyramid tip ends are then pierced to form holes of irregularnon-circular cross section. The foil with the soforrned discrete pyramidsections is thereafter bonded to the metal substrate to form a compositemetal structure, either with the tip end of each pyramid pointingdownwardly toward the substrate surface or with the tip end pointingupwardly away from the substrate surface. In the latter arrangement, thepierced hole through the tip end of each pyramid comprises therestricted opening, the pyramid forms the cavity and the bottom side ofthe unraised section of the foil surrounding each pyramid section isbonded to the substrate. When the pyramid tip ends are arranged to pointdownwardly toward the substrate, the tip edges are bonded to thesubstrate with unbonded gaps between the bonds forming theaforementioned restricted openings and the enclosed space bounded by thesubstrate, shaped foil and the pyramid tip end, form the cavities.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view lookingdownwardly on an up wardly turned pyramid-shaped embodiment of thecomposite metal structure having pierced holes in the pyramid tips.

FIG. 2 is an elevation view taken in cross-section of the FIG. 1structure.

FIG. 3 is a plan view looking downwardly on a downwardly turnedpyramid-shaped embodiment of the composite metal structure havingpierced holes in the pyramid tips.

FIG. 4 is an elevation view taken in cross-section of the FIG. 3structure.

FIG. 5 is an isometric view of a corrugated-serrated sheet compositemetal structure comprising another embodiment of this invention.

FIG. 6 is an elevation view taken in cross-section of a dimpled sheetpierced hole embodiment.

FIG. 7 is an elevation view taken in cross-section of a flat coversheet-venturi type restricted opening embodiment.

FIG. 8 is a graph showing water boiling performance data for the FIG.1-4 pierced hole pyramid, the FIG. 5 corrugated-serrated sheet, and theFIG. 6 dimpled sheet embodiments.

FIG. 9 is a graph showing water boiling performance data for the FIG. 7flat cover sheet-venturi type restricted opening embodiment, and

FIG. 10 is a graph showing fluorot'richloromethane boiling performancedata for the upwardly turned pyramid-shaped cover sheet and flatcover-venturi type restricted opening embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the FIG. l-4 embodiments,cover sheet 11 com prises a thin impervious metal layer of for example0.002-0.030 inch thickness which may be readily deformed into amultiplicity of discrete raised sections. These sections may be in thegeneral shape of pyramids 12 having locally sloping sides 13.Alternatively, they may be conically shaped with a circular base ratherthan the generally square (or rectangular) base of the pyramid typeconfiguration. As illustrated in FIGS. 1-2, pyramids 12 are arranged inparallel rows some of which are oriented at 90 to each other.Alternatively, the individual discrete sections may be arranged in astaggered pattern. The individual discrete raised sections 12 arepreferably separated and surrounded by valley sections 14. At least onerestricted opening 15 is provided in each discrete section 12 and may becircular but is preferably irregular and non-circular. Restrictedopening 15 is preferably in the tip end of the raised section 12.

Cover sheet 11 may for example be formed by the use of a metal diehaving spaced projections of the desired configuration and in thedesired density per square inch of surface area. The die is pressedagainst the undeformed cover sheet which is preferably backed by aresilient layer, as for example rubber foam, to initially form thedesired discrete sections in the sheet by deformation against the matingsurfaces of the die. The restricted openings 15 may be formed in the tipend of each raised section 12 by pressing the die against the nowdeformed cover sheet with additional force so that the tip end of eachdie projection ruptures or pierces the tip end of the mating deformedsection of the cover sheet so as to form irregular non-circular openings15 with jagged edges 16. The openings usually have an xshape orstar-shape in the plan view as illustrated in FIG. 1. The torn edges 16of the openings 15 may for example extend about half-way down thesloping sides 13 of the raised sections 12 (see FIG. 2). The size of theopenings may be varied relative to the size of the cavity formed, bycontrolling how far the projections on the forming die deform the coversheet 11. Also, the size of the restricted opening 15 may be controlledto some extent by selecting the thickness and/or physical properties ofthe cover sheet 12. For example, if the cover sheet is formed of metalfoil, more ductile or annealed foil materials will generally permit theformation of smaller openings than less ductile or work hardened foilmaterials.

The resulting cover sheet 1 1 having a multiplicity of discrete raisedsections 12 separated by valley sections 14 is bonded to the metalsubstrate 17 to form a composite metal structure, either with the tipends pointing upwardly (FIGS. 1-2) or downwardly (FIGS. 3-4). Thebonding may for example be accomplished by sprinkling a layer of brazingmetal powder of desired thickness, e.g. 0.0020.003 inch, onto thesubstrate, positioning the deformed cover sheet 11 thereon andthereafter heating the assembly in a brazing furnace. In the FIG. 1-2embodiment, the bond is formed between the valley sections 14surrounding the raised sections 12 and the smooth top surface substrate17.

The FIG. 3-4 embodiment of the composite metal structure may be formedin the same manner as the FIG. 1-2 embodiment except that the tip endsare positioned against the substrate surface and the bond is between thelatter and the jagged edges 16 of openings 15.

There are however important differences between the composite metalstructures of FIGS. 1-2 and 3-4 with respect to their performance forimproved heat transfer from heated surfaces to boiling liquids. In thetip-up embodiment, the cavities 18 where the vapor bubbles are formedand retained are bounded by the sloping sides 13 of each raised section12 and the top surface of substrate 17. There is preferably at least aminor degree of communication, i.e. fluid communication passages,between at least some of the adjacent cavities 18 by virtue of gaps inthe bond between the valley bottom surface and the substrate 17 topsurface.

In the tip-down embodiment of FIGS. 3-4, the restricted openings areformed by the unbonded gaps 15 between the tip edges 16 and thesubstrate 17 top surface. For example the piercing operation usuallyproduces four comer tears 19 extending a considerable longitudinaldistance along the sloping sides 13, and only the lower portion of thesetears is filled with bonding material 20 during the brazing operation.The upper portion is open and comprises the restricted opening of thetip-down embodiment. The cavities 18 comprise the space bounded by thesubstrate 17 top surface, the shaped cover sheet 11, and the tip ends ofadjacent raised sections 12a and 12b. It should also be noted that thetie-up cavities 18 of FIGS. l-2 are directly beneath the restrictedopenings 15 whereas the tip-down cavities of FIGS. 3-4 are locatedbetween the restricted openings of adjacent raised sections 12a and 12b.FIGS. 3-4 also reveal that unlike the tip-up" embodiment, there arenumerous fluid communication passages between adjacent cavities.

As previously indicated, the composite metal structures of thisinvention have cavities with effective diameters of at least 0.003 inch,and a multiplicity of spaced restricted openings extending through thecover sheet in fluid communication with the cavities, such that theeffective diameter ratio of restricted openings to cavities is less than0.8. The cavities act as nuclei for the growth of many bubbles of theboiling liquid. As the bubbles grow, vapor emerges from the cavitiesthrough the restricted openings due to continued generation of vaportherein, breaks away from the outer surface of the cover sheet, andrises through the liquid. The liquid continues to flow into the cavitiesto replenish a thin liquid layer which is maintained between a trappedvapor bubble and the adjacent metal surface defining the cavity. Theextreme thinness of the liquid film within the cavity is believedprimarily responsible for the strikingly high boiling heat transfercoefiicients achieved with this invention.

The cavities must have effective diameters of at least 0.003 inch andpreferably at least 0.006 inch to permit appreciable bubble growththerein, and the effective diameter ratio of restricted openings tocavities is less than 0.8 and preferably less than 0.7 to retain the hubbles in the cavities for sufficient duration for such growth to occur.The composite metal structure has at least 25 cavities per inch ofsubstrate surface to provide an adequate number of nucleation sites forhigh rate heat transfer.

In general, larger cavity and restrictive opening effective diametersprovide optimum performance (maximum boiling coeflicients) for liquidscharacterized by relatively high Kelvin parameters. The latter isdefined as 2Co- T IA P in units of mils x F where 0'= Surface tension,lbs. force/ft.

T Saturation temperature of boiling liquid corresponding to the vaporpressure of the liquid, R

P Density of vapor, lbs. mass/ft.

A Latent heat of boiling liquid, Btu/lb.

C= Conversion factor, 15.48 Btu mil/ftFX lb. force Conversely, smallercavity and restrictive opening effective diameters provide optimumperformance for liquids characterized by relatively small Kelvinparameters. However the cavity effective diameter of the composite metalstructure of this invention is preferably at least 0.006 inch tominimize possible plugging during the metal substrate-cover sheet metalbonding. These relationships are illustrated by Table A:

TABLE A Kelvin Pressure Optimum Cavity Effective Fluid Parameter -psiaDiameter 1X10")in.

Water 18 1.5 18

In a preferred embodiment of the composite metal structure, the coversheet is shaped to provide fluid communication passage between at leastsome of the cavities adjacently positioned to each other. This is to.assure continuous flow of liquid into active cavities from whichbubbles are continuously emerging even though liquid may not besimultaneously entering the restricting openings of such cavities due tovapor obstruction. Under these circumstances liquid may enter the activecavities from inactive cavities through the fluid communicationpassages. As previously indicated the tip-up embodiment of FIGS. 1-2 ischaracterized by a limited number of such passages whereas the tip downembodiment of FIGS. 3-4 has a large number of vapor communicationpassages 21.

In another preferred embodiment the restricted openings of the compositemetal structure have irregular non-circular cross-sections. The latterwill assure continuous flow of liquid into the restricted opening alongat least part of its perimeter even when vapor is emerging therefrom.This is because the vapor bubbles generally assume a circularconfiguration and do not fill the entire cross-section of an irregularnon-circular configuration. Continuous flow of liquid into a cavityhaving an active vapor nucleation site will of course insure continuousvapor generation therein. Accordingly, if the restricted openings haveirregular non-circular cross-sections the importance of also providingfluid communication passages between at least some adjacent cavities isdiminished and the converse is also true. However, for continuous vapornucleation in a large number of cavities per unit surface area to attainmaximum boiling heat transfer coefficients, restricted openings havingirregular non-circular cross-sections and cavityto-cavity fluidcommunication passages are both employed in the composite metalstructure.

Although the FIGS. 1-4 embodiments are formed with a continuous singlecover sheet positioned over the entire smooth surface substrate, it iscontemplated that the composite metal structures may be formed using adiscontinuous cover sheet. For example, sheets may be folded into aU-shaped configuration and positioned with the open ends against thesubstrate for bonding thereto. Spaced channels could then extend theentire length (or width) of the substrate, and restricted openings tothe channels could be formed in the channel top surfaces. As anotheralternative, the channels may be formed by spaced parallel folds in asingle cover sheet.

In the FIG. 5 embodiment, cover sheet 11 has a series of corrugations 22arranged in longitudinal rows parallel to each other with transverselyseparating valleys 23 between adjacent rows and bonded to the substrate.Each corrugation 22 has transverse slits or serrations 24 to formcorrugation sections with longitudinally alternate sections 25 and 26being transversely displaced in the same direction. That is, alternatesections 25 are displaced to one side of the corrugation 22 centerlineand alternate section 26' are displaced to the opposite side of thecenterline. Moreover, the transversely aligned sections of adjacentcorrugations, e.g. 22a and 22b, are displaced in the same direction, sothat alternate sections 25 of adjacent corrugations are transverselyaligned and alternate sections 26 are transversely aligned. The cavities18 providing the boiling nucleation sites comprise the space covered bythe corrugation sections 25 and 26 so that the partially enclosing wallsare substantially vertical. The restricted openings 15 of this compositemetal structure are formed by transverse slitting and displacement ofcontiguous corrugation sections, so extend substantially the full heightof each cavity with four such openings per cavity.

The FIG. 6 embodiment differs from the FIGS. l-4 embodiment in that thediscrete raised sections 12 are not separated by unraised sections inthe form of parallel rows. Instead the raised sections 12 are dimples incover sheet 11, and may for example be formed by deforming sections ofthe flat sheet into perforation openings of an undersheet in any desiredconfiguration. The restricted openings 15 may be provided in dimples 12by piercing with a sharp pro-jection as for example a pin or punch. Asillustrated, the openings 15 have been formed by downward piercing so asto outwardly and downwardly displace the hole edges into the cavity 18formed by dimples 12. Although only one restricted opening per cavity isshown, it should be understood that multiple openings may be provided ifdesired. Moreover the openings may be provided in the slope or baseportion of dimples instead of the tip (as illustrated).

The FIG. 7 embodiment differs from the FIGS. 1-6 embodiments in thatcover sheet 11 is not deformed to provide discrete sections raised fromthe smooth surface metal substrate 17. lnstead, cavities 18 are formedin the flat cover sheet itself, as by chemically milling therein fromopposite sides of the sheet to provide a restricted opening 15intermediate the sides. It should be noted that chemical millingnormally produces a substantially circular opening. Also, since thebottom side of cover sheet 11 bears directly against the top side ofsubstrate 17 and is bonded thereto, only a relatively small number offluid communication passages between adjacent cavities are likely to beformed. This means that the boiling in a particular cavity is likely tobe intermittent rather than continuous, as occurs for the embodimentshaving either or both irregular noncircular restricted openings and arelatively large number of cavity-to-cavity fluid communicationpassages. In the FIG. 7 embodiment, liquid flows into a particularcavity through its restricted opening, is boiled to form vapor duringwhich period very little (if any) additional liquid enters the cavity,and the resulting vapor bubbles are ejected through the restrictedopening whereupon the sequence is repeated. This embodiment may bebroadly described in terms of the cover sheet being bonded to thesubstrate and provided with a multiplicity of spaced venturi-typeopenings extending through the cover sheet cross-section with the largercross-sectional portion adjacent to the substrate and forming thecavities, and the smaller cross-sectional portion communicating with thecover sheet outer surface and forming the restricted openings.

The invention and its unexpected advantages will be more clearlyunderstood by the following examples.

EXAMPLE 1 A metal die having twenty uniformly spaced pyramid-shapedprojections per lineal inch in each direction (400 projections persquare inch) was pressed against 0.002 inch thick brass foil backed by aresilient rubber surface to initially form the discrete pyramid sectionsin the foil with walls sloping about 45, and then pierce the pyramid tipends to form non circular openings with cracks or tears at the edges inan x-shaped or star-like configuration. The torn edges of these openingsextended about one-half the distance down the angled walls of thepyramids from the tip toward the base and the adjoining valleys. Thepitch P (valley-to valley) was about 0.050 inch and the effectivediameter R of the pierced holes was about 0.008 inch. The width andheight of the pyramid cavities were about 0.039 and 0.012 inchesrespectively. The ratio R/C was about 0.008/0.012 0.67 and the cavitydensity was 400 per square inch. The projections on the die were in theconfiguration of parallel rows with one group oriented at 90 degrees toanother group, so the resulting deformed foil assumed the FIGS. 1-2configuration of metal cover sheet 11.

The deformed and pierced foil was then brazed to a 0.035 inch thicksmooth copper substrate sheet with the pierced tip ends of the pyramidspointing upwardly. This was accomplished by sprinkling a layer ofbrazing metal powder of about 0.002-0.003 inch thickness onto thesubstrate, positioning the deformed metal foil thereon and then heatingthe assembly in a brazing furnace. Accordingly, the valleys between thepyramids were the only areas where metal bonding to the sub strateoccurred. Microscopic examination of the composite metal structurerevealed that there was only a minor degree of interconnection betweenadjacent pyramid cavities due to the proximity of the intervening brazedvalleys and the smooth substrate surface. That is, interconnection onlyexisted in the areas where brazing was incomplete.

EXAMPLE 2 A composite metal structure was prepared from the samematerials as Example 1 and using the same procedure, except that thepierced tip ends of the pyramids were positioned adjacent to the smoothmetal substrate. It will however be apparent from FIGS. 3 and 4 that theeffective diameter ofthe resulting pyramid cavities and the degree offluid intercommunication are quite different from the Example 1structure. The pierced tip ends were brazed to the smooth coppersubstrate and the braze metal substantially filled the central portionopenings. The effective diameter R of the restricted openings is thewidth of the relatively long and narrow torn sections extending upwardlyfrom the brazed central portion as illustrated in FIGS. 3-4, andaveraged about 0.0036 inch. The cavity effective diameter C was about0.016 inch so that the ratio R/C was about 0.22. The pitch P(center-to-center distance of adjacent brazed central portions) was0.050 inch. The width of the pyramid cavities was about 0.035 inch andheight was about 0.016 inch. There was free fluid interconnectionbetween adjacent cavities. The cavity density was about 1,200 per squareinch.

EXAMPLE 3 A composite metal structure was prepared from 0.005 inch thickcopper foil using the same metal die as employed in Example 1 so thatthe cavity density was also 400 per square inch. However, thisrelatively thick foil was softer so that the die failed to punch thesame size openings as in Example 1. Whereas the effective diameter R ofthe latters restricted openings was about 0.008 inch, the openings inExample 3 were only about 0.005 inch. Since the cavity effectivediameter C was about 0.012 inch the ratio R/C was about 0.42. Theresulting deformed and pierced foil was brazed to a 0.250 inch thicksmooth copper substrate plate with the pierced tip ends pointingupwardly in the manner of FIGS 1 and 2 and Example 1. The thickness ofthe brazing powder layer prior to heating was about 0.003 inch.Microscopic examination revealed that many of the narrow comer tears andsome of the crest tears were plugged with filler during brazing. Thiswas due to the relatively small diameter of the pierced openingscombined with an excess of braze metal.

EXAMPLE 4 A composite metal structure was prepared from 0.002 inch thickcopper foil following the same procedure as Example 1, but a thickerlayer of brazing metal powder was used on the substrate, i.e., about0.003-0.004 inch instead of 0.002 inch. Microscopic examination of thecomposite metal structure revealed that too much braze powder had beenused because a majority of pierced openings were completely filled withthe brazed metal.

EXAMPLE 5 A composite metal structure was prepared from an 0.006 inchthick corrugated aluminum sheet and a 0.250 inch thick smooth, flataluminum plate. The pitch P of the corrugations was about 0.165 inch(valley-to-valley) and their height was about 0.065 inch. At

0.125 inch intervals in the longitudinal direction, the corrugationswere transversely notch-slit and alternate sections pushed inwardly. Theresulting openings were about 0.036 inch wide and this dimensionrepresents the effective diameter R of the spaced restricted openingsaccording to this invention. The cavities were about 0.063 incheffective diameter so that the ratio R/C was about 0.57. The cavitydensity was about 48 per square inch. The corrugated and serratedaluminum sheet was then brazed to the flat impervious aluminum plate asthe substrate using a brazing powder layer about 0.003 inch thick, andthe structure closely resembled the FIG. embodiment. It should be notedthat the side walls of the cavities were substantially vertical and theserrated slot openings thereto extended substantially the full height ofthe cavities.

EXAMPLE 6 A composite metal structure of the pierced hole dimpled sheettype (illustrated in FIG. 6) was prepared from 0.004 inch thick copperfoil and 0.250 inch thick smooth flat copper plate. The dimples wereformed by first placing the copper foil over a perforated undersheet,superimposing a rubber sheet on the copper foil and pressing downwardlyon the rubber sheet, thereby deforming sections of the foil into theperforation openings of the undersheet in staggered rows. The pitch P(center-to-center) of the dimples was about 0.125 inch, the width of thedimple base was about 0.10 inch, and the dimple height was 0010-0015inch. The dimpled foil was bonded to the smooth flat copper sheet bysoft soldering at the valley sections. A 0.005 inch diameter needle wasthen used to downwardly pierce 1-3 holes per dimple through the dimpletop. These restricted openings R were about 0.005 inch effectivediameter with the outer edge of the surrounding wall downwardly indentedas illustrated in FIG. 6. The cavity effective diameter C was about0010-0015 inch so that the ratio RIC was 0.3-0.5. The cavity density wasabout 60 per square inch.

EXAMPLE 7 A composite metal structure of the flat cover sheetventuri-shaped restricted opening type (illustrated in FIG. 7) wasprepared from a 0.015 inch thick copper cover sheet and a 0.250 inchthick smooth flat copper substrate. The venturi-type restricted openingswere formed in the cover sheet by chemical milling in parallel rows atspacing of 33 openings per inch in both the longitudinal and transversedirections, so that the density was 1089 holes per inch surface area.The pitch P (center-to-center) was about 0.030 inch. In this particularexperiment, the venturi configuration was formed by chemical millingfrom both the top and bottom and joining in intermediate section of thecover sheet. The opening in the top plane was about 0.013 inch diameterand in the bottom plane about 0.019 inch diameter, with an intermediaterestricted section R of about 0.005-0.008 inch diameter. The cavityeffective diameter was about 0.010 inch so that the ratio R/C was about05-08. In contrast to the composite metal structures of Examples 1-6,the cross-section of the restricted openings was substantially circulardue to the method of formation. The flat cover sheet having theventuri-shaped restricted openings was bonded to the copper substrate bycoating a thin layer of metal brazing paste on the latter, superimposingthe cover sheet and pressing it down with a small weight, then heatingthe assembly in a furnace.

Microscopic examination of cross-sections revealed that the resultingcomposite metal structure was unevenly bonded together. There werenumerous areas where there was no metal bond between the cover sheet andthe substrate and some of these gaps were 0001-0002 inch wide. Thisindicates the presence of many fluid communication passages between atleast some of the cavities adjacently positioned to each other.

EXAMPLE 8 Another composite metal structure was prepared from the samematerials and using the same procedure as Example 7, except that a thinlayer of brazing powder and a porous fibrous layer between the coversheet and the weight were used in an effort to eliminate the unevenbonding and fluid communication passages between adjacent cavities.Microscopic examination of cross-sections revealed that bonding was moreuniform although there were a few gap areas where there was no metalbond. It was therefore concluded that a small number of .fluidcommunication passages between adjacent cavities may have existed.

EXAMPLE 9 Still another composite metal structure was prepared from thesame materials and using the same procedure as Example 7 and 8, exceptthat a. more uniform pressure holddown arrangement for the cover sheetwas used. Microscopic examination of cross-sections indicated virtuallycomplete and uniform brazing with only about 1.2 percent of the possiblejoint area unfilled with braze metal.

The Example 8 and 9 composite metal structures were studied to determinewhether liquid could freely enter the isolated cavities. This wasimportant to determine because the liquid reentry path employed by priorart nucleate boiling surfaces, i.e., fluid communication passages fromadjacent inactive cavities, had been virtually eliminated. Also, thecircular cross-section of the restricted openings to these cavities inthe Example 8 and 9 composites are substantially filled by the vaporbubbles emerging therefrom, so that very little area is open for liquidingress (in contrast to the irregular noncircular cross-sections of theExample l-6 openings prepared by metal piercing).

In this investigation, a reagent was used which reacted with the coppersurfaces of the Example 7 and 8 composite structures to produce acoating of black CuO. The reagent, consisting of sodium hypochlorite ina basic solution, was added in concentrated solution to the liquid poolin which the composite structure had been immersed for boiling tests.The reagent was added after the stable boiling conditions had beenattained. About 15 minutes was required to blacken the copper surfaces,whereupon the pool was dumped and the composite washed immediately infresh water. Since the reagent could only act on surfaces in contactwith the liquid phase, any areas which remain dry during boiling shouldnot show the presence of the oxide layer.

When the Example 8 composite was cross-sectioned, oxide was discoveredinside all of the cavities including those in areas where the brazedjoint was good so as to isolate a given cavity from adjacent cavities.The oxide layer appeared to be as thick inside these isolated cavitiesas in the unisolated cavities, indicating that liquid had easilypenetrated the former through the circular restricted openings.

It was also observed that the restricted openings of the Example 8composite were not perfectly circular and were not smooth-edged. Todetermine whether a smooth-edged perfectly circular opening would resultin a dry cavity which would be sealed from liquid by the vapor bubble, ablock of 36 restricted openings of the Example 9 composite were reamedby insertion of a 0.008 inch diameter wire. After the boiling test inthe CuO-containing liquid, the Example 9 composite was cross-sectionedand the oxide coating was found in equal thickness in the cavitiesjoined to the reamed and unreamed openings. These tests demonstrate that(l) interconnections between adjacent cavities are not essential, and(2) smooth-edge perfectly circular openings do not prevent liquidingress to the sub-surface cavities.

EXAMPLE 10 The Example l6 composites were each tested to determine thewater boiling performance at various heat fluxes and 1 atmospherepressure, and the results, of these tests are plotted in the FIG. 8graph. The numerals identifying the various curves correspond to theaforelisted examples, and the dotted line indicates the performance of asmooth surface for comparison. It will be readily apparent from FIG. 8that each of the Example 1-6 composites is greatly superior to a smoothsurface. That is, the temperature difference (A T) required for a givenheat flux using the Example l-6 composites is only a small fraction ofthat required using a smooth surface. The performance of thesecomposites may be compared at the same heat flux, as for example 20,000Btu/hr.- F., and is summarized in' EXAMPLE 11 The Example 7-9 compositeswere also tested to determine the water boiling performance of the flatcover sheet-venturi type restricted opening embodiment at various heatfluxes and 1 atmosphere pressure. The results of these tests are plottedin FIG. 9 graph along with the smooth surface (dotted line) forcomparison. It is also apparent that each of the Example 7-9 compositesis greatly superior to a smooth surface as indicated by the 20,000Btu/hr. F. heat flux boiling coefficients in Table C.

TABLEC cavity effec- Boiling inter tive Cav- Co- Comcom- Cavdiameters(ll0) ities efficient posmuniity in. restricted per Btu/hrite cation (C)opening(R) R/C in ft"-F 7 Many 10 5-8 0.5-.8 1,089 5,500 8 Small 10 5-805-8 1,089 7,100

number 9 None 10 5-8 0.5-.8 1,089 5,300 Smooth None None None None None620 EXAMPLE 12 The outstanding performance of the composite metalstructures for boiling liquids at room temperature was demonstratedusing Examples 4, 7 and 9 to boil fluorotrichloromethane at oneatmosphere (boiling point of 738 F.) and various heat fluxes. Theperformances are plotted in FIG. 10 and a smooth surface is included forcomparison. A comparison at heat flux of 20,000 Btu/hr. F. is summarizedin Table D.

Comparison of the boiling coefficients in Tables B-D with thecorresponding values for the prior art high performance nucleate boilingsurfaces reveals that the performances are similar. However, based onequivalent surface area, the cost to manufacture the pyramidshapedcomposite metal structures of this invention is only about one-thirdthat of the deformed groove type described in Kun et al. US. Pat. No.3,454,081.-

Although preferred embodiments of this invention have been described indetail, it is contemplated that modifications of the structures and theforming method may be made and that some features may be employedwithout others, all within the spirit and scope of the invention.

What is claimed is:

l. A boiling to heat transfer surface formed from a composite metalstructure comprising an impervious smooth surface metal substrate and animpervious metal cover sheet bonded to said metal substrate and havingat least 25 cavities per inch of substrate surface with each cavityhaving an effective diameter of at least 0.003 inch, and a multiplicityof spaced restricted openings extending through said cover sheet influid communication with said cavities and having effective diametersuch that the effective diameter ratio of restricted openings tocavities is less than 0.8.

2. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is shaped toform a multiplicity of discrete sections raised from said metalsubstrate as said cavities.

3. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is shaped toform a multiplicity of discrete sections raised from said metalsubstrate as said cavities, with each raised section surrounded byunraised sections bonded to said cover sheet.

4. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is shaped toform a multiplicity of discrete sections raised from said metalsubstrate as said cavities, and said restricted openings have irregularnon-circular cross-sections.

5. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein each cavity has an effectivediameter of at least 0.006 inch.

6. A boiling to heat transfer surface formed from a composite metalstructure according to claim 4 wherein said discrete sections arepyramid-shaped with the tip end of each pyramid pointing downwardlytoward the substrate surface, and at least one of said restrictedopenings extends through each tip end.

7. A boiling to heat transfer surface formed from a composite metalstructure according to claim 4 wherein said discrete sections arepyramid-shaped with the tip end of each pyramid pointing upwardly awayfrom the substrate surface and at least one of said restricted openingsextends through each tip end.

8. A boiling to heat transfer surface formed from a composite metalstructure according to claim 2 wherein said cover sheet is shaped toprovide fluid communication passages between at least some of saidcavities adjacently positioned to each other.

9. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is metal foilshaped to form a multiplicity of discrete pyramid sections raised fromsaid metal substrate as said cavities with the tip end of each pyramidpointing upwardly away from the substrate surface, holes of irregularnon-circular cross-section pierced through said tip end of each pyramidcomprise said restricted openings, and the unraised sections of saidfoil surround said pyramid sections with the foil bottom side bonded tothe substrate.

10. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is metal foilshaped to form a multiplicity of discrete pyramid sections raised fromii'is i i81l $ieilf8ifh th fi$ %2 l ii pyramid, being positioned withsaid tip ends pointing downwardly toward said substrate and the tip endbonded thereto with unbonded gaps between said tip end and saidsubstrate forming said restricted openings, and the space bounded bysaid substrate, the shaped foil and the pyramid tip ends forming saidcavities.

1 1. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet comprises asheet having a series of corrugations arranged in longitudinal rowsparallel to each other with separating valleys between adjacentcorrugations bonded to said substrate, each corrugation havingtransverse slits to form corrugation sections with longitudinallyalternate sections in each corrugation being transversely displaced inthe same direction and the transversely aligned sections of adjacentcorrugations being displaced in the same direction.

12. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is bonded tosaid substrate and is provided with a multiplicity of spaced venturitypeopenings extending through the cover sheet crosssection with the largercross-sectional portion adjacent to said substrate and forming saidcavities, and the smaller cross-sectional portion communicating with thecover sheet outer surface and forming said restricted openings.

13. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein the effective diameter ratio ofrestricted openings to cavities is less than 0.7.

14. A method for forming a boiling to heat transfer surface formed froma composite metal structure comprising: providing a die having at least25 spaced pyramid shaped projections per inch of die surface area andpressing same against a thin metal foil backed by a resilient surface toinitially form discrete pyramid shaped sections in the foil; piercingthe tip ends to form holes of irregular non-circular cross-section; andbonding the shaped foil to a smooth surface metal substrate.

15. A method according to claim 14 wherein the pierced pyramid tip endsare positioned upwardly away from said substrate, and the unraisedsections of the foil bottom side are bonded to said substrate with saidholes forming restricted openings to cavities formed by said pyramidshaped sections such that. the cavities have an effective diameter of atleast 0.003 inch and the effective diameter ratio of said restrictedopenings to said cavities is less than 0.8.

16. A method according to claim 14 wherein the pierced pyramid tip endsare positioned downwardly toward said substrate and bonded thereto withunbonded gaps between said tip ends and said substrate formingrestricted openings to cavities formed by said pyramid shaped sectionssuch that. the cavities have an effective diameter of at least 0.003inch and the effective diameter ratio of said restricted openings tosaid cavities is less than 0.8.

2. A boiling to heat transfer surface formed from a composite metalstructure according to claim 1 wherein said cover sheet is shaped toform a multiplicity of discrete sections raised from said metalsubstrate as said cavities.
 3. A boiling to heat transfer surface formedfrom a composite metal structure according to claim 1 wherein said coversheet is shaped to form a multiplicity of discrete sections raised fromsaid metal substrate as said cavities, with each raised sectionsurrounded by unraised sections bonded to said cover sheet.
 4. A boilingto heat transfer surface formed from a composite metal structureaccording to claim 1 wherein said cover sheet is shaped to form amultiplicity of discrete sections raised from said metal substrate assaid cavities, and said restricted openings have irregular non-circularcross-sections.
 5. A boiling to heat transfer surface formed from acomposite metal structure according to claim 1 wherein each cavity hasan effective diameter of at least 0.006 inch.
 6. A boiling to heattransfer surface formed from a composite metal structure according toclaim 4 wherein said discrete sections are pyramid-shaped with the tipend of each pyramid pointing downwardly toward the substrate surface,and at least one of said restricted openings extends through each tipend.
 7. A boiling to heat transfer surface formed from a composite metalstructure according to claim 4 wherein said discrete sections arepyramid-shaped with the tip end of each pyramid pointing upwardly awayfrom the substrate surface and at least one of said restricted openingsextends through each tip end.
 8. A boiling to heat transfer surfaceformed from a composite metal structure according to claim 2 whereinsaid cover sheet is shaped to provide fluid communication passagesbetween at least some of said cavities adjacently positioned to eachother.
 9. A boiling to heat transfer surface formed from a compositemetal structure according to claim 1 wherein said cover sheet is metalfoil shaped to form a multiplicity of discrete pyramid sections raisedfrom said metal substrate as said cavities with the tip end of eachpyramid pointing upwardly away from the substrate surface, holes ofirregular non-circular cross-section pierced through said tip end ofeach pyramid comprise said restricted openings, and the unraisedsections of said foil surround said pyramid sections with the foilbottom side bonded to the substrate.
 10. A boiling to heat transfersurface formed from a composite metal structure according to claim 1wherein said cover sheet is metal foil shaped to form a multiplicity ofdiscrete pyramid sections raised from said metal substrate with holes ofirregular non-circular cross-section pierced through the tip end of eachpyramid, being positioned with said tip ends pointing downwardly towardsaid substrate and the tip end bonded thereto with unbonded gaps betweensaid tip end and said substrate forming said restricted openings, andthe space bounded by said substrate, the shaped foil and the pyramid tipends forming said cavities.
 11. A boiling to heat transfer surfaceformed from a composite metal structure according to claim 1 whereinsaid cover sheet comprises a sheet having a series of corrugationsarranged in longitudinal rows parallel to each other with separatingvalleys between adjacent corrugations bonded to said substrate, eachcorrugation having transverse slits to form corrugation sections withlongitudinally alternate sections in each corrugation being transverselydisplaced in the same direction and the transversely aligned sections ofadjacent corrugations being displaced in the same direction.
 12. Aboiling to heat transfer surface formed from a composite metal structureaccording to claim 1 wherein said cover sheet is bonded to saidsubstrate and is provided with a multiplicity of spaced venturi-typeopenings extending through the cover sheet cross-section with the largercross-sectional portion adjacent to said substrate and forming saidcavities, and the smaller cross-sectional portion communicating with thecover sheet outer surface and forming said restricted openings.
 13. Aboiling to heat transfer surface formed from a composite metal structureaccording to claim 1 wherein the effective diameter ratio of restrictedopenings to cavities is less than 0.7.
 14. A method for forming aboiling to heat transfer surface formed from a composite metal structurecomprising: providing a die having at least 25 spaced pyramid shapedprojections per inch2 of die surface area and pressing same against athin metal foil backed by a resilient surface to initially form discretepyramid shaped sections in the foil; piercing the tip ends to form holesof irregular non-circular cross-section; and bonding the shaped foil toa smooth surface metal substrate.
 15. A method according to claim 14wherein the pierced pyramid tip ends are positioned upwardly away fromsaid substrate, and the unraised sections of the foil bottom side arebonded to said substrate with said holes forming restricted openings tocavities formed by said pyramid shaped sections such that the cavitieshave an effective diameter of at least 0.003 inch and the effectivediameter ratio of said restricted openings to said cavities is less than0.8.
 16. A method according to claim 14 wherein the pierced pyramid tipends are positioned downwardly toward said substrate and bonded theretowith unbonded gaps between said tip ends and said substrate formingrestricted openings to cavities formed by said pyramid shaped sectionssuch that the cavities have an effective diameter of at least 0.003 inchand the effective diameter ratio of said restricted openings to saidcavities is less than 0.8.